Radio Frequency Identification (RFID) systems are used in a variety of ways in product tracking, supply chain management, and numerous logistical operations. Applications of RFID tags include replacement of bar codes in inventory management of consumer items, tracking of books in libraries or bookstores, shipping container and truck/trailer tracking, and livestock tracking. In the automotive field, RFIDs are used in car keys to activate vehicles and for tire tracking. Transport payments are enabled by the use of RFID smartcards.
A typical RFID system consists of the inclusion of one or more small inexpensive tags that contains transponders with a digital memory chip that is given a unique product code. In the case of passive tags, a base station (or reader) emits a signal activating the RFID tag to cause it to emits a signal. The reader can then receive this signal and decode the data encoded therein. As is know in the RFID art, RFID readers transmits RF power to RFID tags. RFID tags are interrogated by, and respond to, RFID readers utilizing a radio-frequency forward link and a backscatter return link. Some RFID tags contain a demodulator which is used to recover a timing (or clock) signal from the signal received from the RFID reader. The recovered clock signal is then utilized to generate a value to control a digitally-controlled oscillator that provides clock signal used in the backscatter return link.
RFID systems have been allocated bands of operation at particular frequencies. Low-frequency (125 kHz) RFID tags can be used globally without a license. Additional, 900 MHz tags are typically used in warehousing and shipping, while the lower frequencies (125 kHz, 13.56 MHz) are generally used for inventory or shelving operations.
Low-frequency and higher-frequency RFID systems each have their advantages and disadvantages. For example, it is generally easier and cheaper to generate RF power at lower frequencies. Since many applications require a low-cost RFID tag system, low frequencies are commonly used. However, low frequency systems require physically larger antennas and can result in signal propagation to unwanted areas. Signal processing to correct these phenomena is possible but would make the tags too expensive. The low carrier frequency also puts a ceiling on the allowable data rate. Without the use of more intensive and expensive signal processing techniques it is difficult to approach a data transmission bit rate of 1 kbps per kHz, so a 125 kHz system would top off at around 100 kbps data rate transfer, which though in some applications would be more than adequate, in others would be a limitation.
There is a band of very high frequencies in the 57-64 GHz range (“60 GHz band”) that is located in the millimeter-wave portion of the electromagnetic spectrum and has been largely unexploited for commercial wireless applications. This spectrum is unlicensed by the FCC in the United States and by other bodies world-wide. In addition to the higher-data rates that can be accomplished in this spectrum, energy propagation in the 60 GHz band has unique characteristics that make possible many other benefits such as excellent immunity to interference, high security, and frequency re-use. However, RFID tags designed for receiving millimeter frequency signals are more expensive than tags designed for receiving low frequency signals. As such, what is needed is an RFID system that can take advantage of the benefits of both low and high frequency signals.